1. Field of the Invention
This invention relates to a semiconductor storage device which is electrically rewritable.
2. Description of the Related Art
A nonvolatile memory, which is electrically writable and erasable, generally employs a structure which includes a recording transistor provided with a floating gate under a gate and a selecting transistor connected to the recording transistor. The floating gate has a structure in which electrically insulated electrodes are provided. In the structure, when a high voltage is applied to a drain and a gate, it is possible to store (write) an electron in the floating gate or to emit (erase) the electron stored in the floating gate.
A high voltage required in writing or erasing is generally about 20 V. As the high voltage, an optimum voltage is supplied such that a high voltage outputted by a booster circuit provided inside an IC is limited in a voltage by a limiter circuit so as not to be equal to or higher than a predetermined voltage to execute writing or erasing of data to the recording transistor.
Up to now, the limiter circuit outputs by one value a high voltage for executing writing or erasing of data to the recording transistor, and uses a surface breakdown voltage characteristic of a high withstand voltage MOS transistor.
A surface breakdown voltage of the high withstand voltage MOS transistor is generally about 20 V, and thus being most suitable for a voltage required for writing or erasing of data to the recording transistor.
For the purpose of measuring, for example, as an inspection before shipment of a product, electric characteristics of the recording transistor and the selecting transistor adapted to the recording transistor, a voltage is applied to the recording transistor and the selecting transistor. As the voltage required for this purpose, there is used a power supply voltage supplied from the outside of the IC or the limiter circuit output voltage for executing writing/erasing of data to the recording transistor.
Up to now, the limiter circuit is a circuit which supplies by one value a high voltage required for writing/erasing of data to the recording transistor. The high voltage supplied to the recording transistor and the selecting transistor adapted to the recording transistor uses an output of one value outputted in the limiter circuit in any purposes such as execution of writing/erasing of data to the recording transistor and execution of electric characteristic evaluation of the recording transistor and the selecting transistor.
The nonvolatile memory, which is electrically writable and erasable, is composed of the recording transistor provided with the floating gate under the gate and the selecting transistor adapted to the recording transistor. A gate of the selecting transistor is connected to a gate of the selecting transistor of a plurality of bits and is also used as a word line. In the word line, there is used polysilicon wiring in many cases. Below the polysilicon wiring, a poly-gate field transistor is formed and a source/drain of the poly-gate field transistor serves as a diffusion layer of the source/drain of the selecting transistor of the adjoining bit.
A reverse voltage of the poly-gate field transistor is designed so as to be higher than the high voltage supplied by the limiter circuit output, and therefore the poly-gate field transistor is not turned on.
However, semiconductor manufacturing process has progressed in miniaturization, so that it is difficult to increase a reverse voltage of the poly-gate field transistor.
The reverse voltage of the poly-gate field transistor is greatly influenced by a channel length of the poly-gate field transistor, that is, the distance from the diffusion layer of the source/drain of the adjoining selecting transistor. In recent miniaturizing process, this distance is shortened and the reverse voltage is further lowered. When the high voltage supplied by the limiter circuit output is higher than the reverse voltage of the poly-gate field transistor, the poly-gate field transistor is turned on. Therefore, there arises such a potential state that the bit line of the adjoining bit is short-circuited, resulting in a problem. To a gate of the poly-gate field transistor as the word line, at the time of writing/erasing of data to the recording transistor, a high voltage supplied by the limiter circuit output is applied, and then the transistor is turned on. However, since the source/drain of the adjoining selecting transistor as the source/drain both have the same potential or a high impedance, no current flows, resulting in no problem.
However, when the electric characteristics of the recording transistor and the selecting transistor are evaluated, the source/drain of the adjoining selecting transistor do not necessarily have the same potential, which causes a problem in that the electric characteristics can not be evaluated.
In order to solve the above-mentioned problems, the present invention has been made and has an object to provide a semiconductor storage device capable of outputting a high voltage supplied by a limiter circuit output as two values consisting of: a high voltage for executing writing/erasing of data to a recording transistor; and a high voltage for evaluating, as inspection before shipment of a product, for example, electric characteristics of the recording transistor and a selecting transistor, and of selectively outputting one voltage value out of the two values from the limiter circuit in synchronism with a signal generated by a timing circuit inside an IC or a signal given inside the IC via a terminal from the outside of the IC. In order to attain such object of the invention, the following measures are taken.
According to the present invention, there is provided a semiconductor storage device which is electrically writable and erasable, including: a booster circuit for boosting a power supply voltage supplied to an IC; and a limiter circuit having a function of performing voltage limitation of a high voltage which is a booster circuit output so as not to be boosted to a predetermined voltage or more, characterized in that the limiter circuit is capable of outputting two values of high voltage and the circuit is capable of selecting one voltage value out of the two values in synchronism with a signal generated by a timing circuit inside the IC or a signal given inside the IC via the terminal from the outside of the IC, to output the one of two voltage values.
According to the present invention, the high voltage which can be outputted from the limiter circuit is composed of two potentials having different voltage values, consisting of a high voltage for executing writing/erasing of data to the recording transistor and a high voltage for evaluating electric characteristics of the recording transistor and the selecting transistor.
The former high voltage for executing writing/erasing of data to the recording transistor is a voltage which is higher than a reverse voltage of a poly-gate field transistor formed on a word line, and which is required to move an electron to a floating gate provided in the recording transistor.
The latter high voltage for evaluating the electric characteristics of the recording transistor and the selecting transistor is a voltage which is lower than the reverse voltage of the poly-gate field transistor formed on the word line, and which does not turn on the poly-gate field transistor even if the high voltage is applied to the word line.
By switching those two high voltages according to the purpose thereof, it is possible to realize the proper function with respect to any operation of the IC.
According to the present invention, the limiter circuit that can output two kinds of high voltages, which are capable of being selectively switched, includes a high withstand voltage MOS transistor and a low withstand voltage MOS transistor, and which uses a difference between the withstand voltages of surface breakdown voltages of the respective MOS transistors.
The limiter circuit can output two values of the high voltages obtained by the two MOS transistor characteristics having different withstand voltages, and performs voltage limitation by connecting one of MOS transistors to an output of the booster circuit in synchronism with the signal generated by the timing circuit inside the IC or the signal given inside the IC via the terminal from the outside of the IC.
There is employed a circuit structure in which a switch adapted to the high withstand voltage MOS transistor can be eliminated. With this structure, it is possible to contribute to layout saving. The limiter circuit output voltage is determined by a surface breakdown characteristic of the high withstand MOS transistor by closing a switch of the low withstand voltage MOS transistor. On the contrary, when the switch is turned on, the output of the booster circuit is connected with the low withstand voltage MOS transistor and the high withstand voltage MOS transistor. However, since the limiter circuit output voltage is determined by the lower withstand voltage, it is determined by a surface breakdown characteristic of the low withstand MOS transistor.
There is also employed a circuit structure in which a switch adapted to the low withstand voltage MOS transistor can be eliminated. With this structure, it is possible to contribute to further layout saving. The high withstand voltage MOS transistor has a structure which also serves as the switch of the low withstand voltage MOS transistor. In the case where the high withstand voltage MOS transistor is opened, the limiter circuit output voltage is determined by the surface breakdown characteristic of the low withstand voltage MOS transistor. In the case where the high withstand voltage MOS transistor is closed, it is possible to perform voltage limitation thereof by the surface breakdown characteristic of the transistor itself.